A single stage compressor with an overhung rotor for process gas having one end of the compressor rotor exposed to atmospheric pressure and the other rotor end exposed to suction pressure can incur an extremely large unbalanced load on the thrust area of the rotor as a result of the imposed pressure differential between atmosphere and suction pressures. At low operating speeds, such as at startup or shutdown, the imposed loading is known to exceed the load factor for which the thrust bearing was selected. For that reason, it has been necessary to improvise in order to allow low speed operation when excess thrust load is being incurred.
A conventional approach toward resolving the foregoing has been to increase the load capacity of the thrust bearing sufficient to withstand the load level which the bearing incurs. This approach is disadvantageous because the larger bearing requires higher oil flow and has higher power losses at design speed. Another approach has been to reduce the pressure differential by venting the process gas from the compressor to reduce the overall internal pressure to an acceptable level. This approach has been likewise unsatisfactory in that it will result in a considerable waste of process gas during compressor operation. Still another technique has been to counteract the high pressure differential with appropriately directed high pressure oil at a pressure which may be significantly higher than the process gas. This approach, in the past, has tended to complicate and enlarge the seal oil system.
It is an object of the present invention to provide a thrust balancing system for a compressor which will be effective to avoid excessive thrust loadings during periods of low rotor speed.
It is another object of the invention to accomplish the foregoing object without increasing thrust bearing size, or causing increased thrust bearing design complexity. Further it is an object of the invention to avoid increasing oil pressures or having to vent process gas.
It is yet a further object of the invention to utilize high pressure seal fluid during periods of low speed operation and also to utilize lower pressure lubrication fluid during periods of normal compressor operation within the thrust bearing housing.